The present invention relates to apparatus and methods for detecting an incipient fire condition and particularly relates to an incipient fire detector and methods of detection which utilize the shift in particulate size distribution and particularly the ratio of the outputs of sensors sensing particulates of different sizes as an indication of an incipient fire condition.
Fire detection devices and systems available today embody a wide variety of principles. Most are based on the presence of flame, smoke, a preselected temperature level, or the like. Many of these detect a fire only after combustion actually occurs. Others provide for detection of an incipient fire condition. Detectors of the latter type detect the increase in the submicron particulates given off by combustible materials when heated but before the actual onset of combustion. Examples of incipient fire detectors are described and illustrated in U.S. Pat. No. 3,953,844, and U.S. Pat. No. 4,035,788, both of common assignee herewith. In the former patent, there is disclosed an incipient fire detector having a collector for particulates of a specified size, directing them to a sensor having an output which is a function of the increase in mass of the particulates sensed. The rate of change of the output in comparison with a predetermined value gives an indication of an incipient fire condition.
Discrimination among the various particulates in a fluid which indicate an incipient fire condition and those that do not is an important aspect in incipient fire detection and the prevention of false alarms. The system disclosed in U.S. Pat. No. 3,953,844 achieves a degree of discrimination in that only particles smaller than a predetermined size are presented to and detected by the sensor. However, there are situations where significant amounts of particulates even of that small size are created, and which are different from or exceed in concentration those normally present in the atmosphere in that size range and which could set off an alarm. Such alarms would not necessarily represent a developing combustion situation and would constitute false alarms. The disclosure of U.S. Pat. No. 4,035,788 is similar in this respect.
Furthermore, the proximity of the fire detector to the source of particles it is detecting, in that type of detector where particulate concentration is being detected, is often a factor in the efficacy of such fire detectors. For example, where the rate of change of the mass concentration of particulates is being measured, it is preferred that the detector be located in close proximity with the source of the hazardous condition. Otherwise, fire may break out before the mass concentration has reached the activation level at a remote alarm, and the purpose of the incipient fire detector is defeated. Because it is usually not known precisely where a hazardous condition will arise, a number of incipient fire detectors of this type are required to be spaced about the area being monitored. Obviously, this is not economical.